In a conventional air hoist employing a ball-screw mechanism, the drum of the hoist is rotated through the ball-screw mechanism by a piston which is reciprocated within a cylinder by means of a compressed fluid. In this conventional structure, the cylinder and drum are made separately so that the cylinder must have sufficient length to accommodate the reciprocating distance of the piston. This increases not only the size of the entire hoist, but also the number of component parts, and thus makes it difficult to obtain a small and low-cost hoist.
Several attempts have been made to use the drum itself as the cylinder to receive the piston which is operated by the high-pressure fluid. These attempts, however, have created the problem that the pressure of the working fluid must be substantially increased in order to overcome the large rotational and reciprocal resistance to which the piston is subjected when the drum turns.
In addition, the control device for regulating the operation of conventional air hoists is able to perform its control function under any of the lifting, lowering or stopping conditions so long as the load being hoisted is maintained constant. However, the know control devices have been unable to keep the hoisted load in a balanced condition when the load changes, unless the control pressure is appropriately adjusted with each change.
Thus, it is an object of this invention to provide an improved air hoist, and a control device therefor, which overcomes the aforesaid shortcomings.